The present invention relates in general to lighting units incorporating an arc discharge lamp with an incandescent filament and more specifically to providing power to the incandescent filament whenever the arc discharge lamp is not able to provide light.
Light sources are known in the art which combine an arc-discharge lamp with an incandescent filament. The filament provides standby illumination during various times that the arc lamp is not producing light. During normal operation of the more efficient arc discharge lamp, the filament serves as a resistive ballast for the arc lamp. An example of this type of light source is Peil et al., U.S. Pat. No. 4,350,930, of common assignment, which is hereby incorporated by reference.
Arc discharge lamps exhibit three distinct operating states, namely the breakdown mode, the glow discharge mode and the arc mode. Thus, operating circuits for arc lamps energize the arc lamp in a manner which is appropriate to the instantaneous operating state of the lamp. For example, to start the arc lamp from the breakdown mode, high voltage pulses are supplied to the arc lamp in order to cause electrical breakdown of the gas contained in the arc lamp. The arc lamp is usually operated at a somewhat lower voltage during the glow mode when the gas warms up and its voltage drop decreases. In the arc mode, an even lower voltage (either AC or DC) is supplied to sustain a stable, low voltage arc. An AC arc-mode voltage can be supplied to the arc lamp by a polarity-reversing bridge.
The voltage required to cause a breakdown of the gas when the gas temperature is above room temperature is proportional to the temperature of the gas. A hot restart occurs when an arc discharge is interrupted for more than about one millisecond and an attempt is made to re-establish the arc while the gas is still hot. If the gas is at a temperature above a certain threshold, the operating circuit will be unable to cause an electrical breakdown of the gas. In that case, the operating circuit waits a predetermined time for the gas to cool and in the meantime energizes the filament for standby lighting.
The high voltage pulses used to start an arc are typically supplied by a starting oscillator. The starting oscillator also usually supplies power to the filament during the standby period and in some cases during a filament warm-up period prior to a cold start.
A starting oscillator generating high voltage pulses has been found to be undesirable because of large amounts of electromagnetic interference (EMI) produced. An improvement in EMI production is realized by employing a starting oscillator which operates intermittently rather than continuously during starting. This "low duty cycle" oscillator allows a lower power (and hence less expensive) semiconductor switch to be used in the oscillator. However, since the oscillator operates for only a small percentage of the time during hot restart, it can no longer be used to provide filament power. Consequently, filament power is provided either by an additional power switch used to turn on the filament when the oscillator is off, or by holding the oscillator power switch (connected in series with the filament) in its on state continuously for standby lighting when the oscillator is not operating. However, these solutions negate the economy of the low duty cycle oscillator since they require either an additional semiconductor switching element or a higher power-rating for the oscillator power switch.
Accordingly, it is a principal object of the present invention to provide a combination arc lamp and filament lighting system having low cost and low EMI.
It is another object to provide power to the incandescent filament during hot restart and other times without increasing the cost of the low duty cycle oscillator and without adding any extra power semiconductor switches.